Method for testing wave transmission rate of material under low frequency based on Luneberg lens

Abstract

The invention discloses a low-frequency material wave transmission rate testing method based on a luneberg lens, and belongs to the technical field of radar material wave transmission rate test.The method comprises the steps that a testing clamp, a transmitting luneberg lens antenna and a receiving luneberg lens antenna are placed on a testing platform, and a to-be-tested material sample is placed on the testing clamp; a to-be-tested material sample is placed between the transmitting luneberg lens antenna and the receiving luneberg lens antenna, the test distance is adjusted, and the center of the to-be-tested material sample is located at the focus position of the transmitting luneberg lens antenna and the receiving luneberg lens antenna; the transmitting luneberg lens antenna and the receiving luneberg lens antenna are respectively connected with the vector network analyzer; an electromagnetic wave is transmitted by a transmitting antenna, is focused by a transmitting-side luneberg lens to form a parallel wave, penetrates through a to-be-tested material sample, enters a receiving-side luneberg lens to be focused, and is received by a receiving antenna; and comparing the transmitting signal parameter of the transmitting antenna with the receiving signal parameter of the receiving antenna by using the vector network analyzer, and evaluating the wave transmission rate of the to-be-tested material sample at low frequency.

Description

Technical Field

[0001] The present invention belongs to the technical field of radar material transmittance testing, and in particular relates to a method for testing material transmittance at low frequencies based on a Luneburg lens. Background Art

[0002] When testing material transmittance, focusing lens antennas are often used for high-frequency testing (such as microwaves and millimeter waves) due to their strong directionality and focusing capabilities. By precisely focusing electromagnetic waves onto the surface of the material under test, lens antennas reduce signal diffusion, thereby improving signal strength and measurement accuracy. This focusing effect is particularly important in high-frequency testing, as high-frequency signals are prone to attenuation and distortion during propagation. Lens antennas effectively concentrate the signal, thereby improving transmittance measurement accuracy. Furthermore, lens antenna designs typically provide strong gain, enabling clear, stable signals to be obtained even in complex electromagnetic environments, thereby enhancing the reliability of test results.

[0003] While focusing lens antennas excel in material transmittance testing at high frequencies, their application in the lower frequency bands (300MHz to 2GHz) faces significant limitations. The longer wavelengths of low-frequency electromagnetic waves necessitate a larger and heavier lens antenna to effectively focus signals at these wavelengths. This makes lens antennas bulky and heavy at low frequencies, making them difficult to transport, install, and use. This also limits their applicability in space-constrained environments, particularly those requiring rapid deployment. Consequently, the use of traditional focusing lens antennas at low frequencies, particularly in real-world testing, becomes impractical.

[0004] Existing technologies face significant challenges when testing material transmittance in low-frequency bands, primarily due to the bulky and heavy weight of focusing lens antennas due to their longer wavelengths in this frequency band. These issues not only increase the difficulty of transporting, installing, and using lens antennas, but also limit their application in environments that require rapid deployment or are space-constrained. In addition, the high manufacturing costs of low-frequency focusing lens antennas, including material costs and complex processing, limit their widespread deployment in economically sensitive applications. The curved structure of the lens antenna also makes integration with the feed system difficult, further increasing the complexity and cost of the system. The complexity of the manufacturing process drives up costs and limits its competitiveness in modern communication systems. Therefore, the main challenges faced by existing technologies in testing material transmittance in low-frequency bands are that the lens antennas are too large, too heavy, costly, difficult to integrate, and difficult to manufacture.

[0005] Low-frequency wave transmission testing is crucial in a variety of fields. In communications, many long-distance communication systems, such as shortwave communications and some satellite communication bands, operate at low frequencies. Understanding a material's wave transmission performance in these frequency bands helps optimize the design of communication equipment antenna covers and base station protective materials, reducing signal transmission loss and improving communication quality and coverage.

[0006] In terms of radar applications, low-frequency radar has unique advantages in detecting long-distance targets and penetrating some obstacles (such as clouds and vegetation). Testing the low-frequency wave transmittance of materials can provide key data for the research and development of radar antenna protection materials and stealth materials, thereby enhancing radar performance and survivability.

[0007] Regarding electromagnetic shielding and protection of electronic equipment, low-frequency electromagnetic waves are common in complex electromagnetic environments. Determining the wave transmission characteristics of materials in the low-frequency band can accurately design the electronic equipment casing, internal shielding structure, etc., ensure the stable operation of the equipment, and reduce the impact of electromagnetic interference on the equipment function. It plays an indispensable supporting role in the manufacture of electronic equipment in many industries such as military, aerospace, and industrial automation.

[0008] Currently, radar-transmitting material transmittance testing primarily focuses on high-frequency bands (2-40 GHz), with detailed specifications and the use of focusing lens methods to improve accuracy. However, testing in the low-frequency band (300 MHz-2 GHz) presents significant challenges. Due to the long wavelengths of low-frequency electromagnetic waves, large antennas and equipment are required, resulting in bulky test systems and difficult deployment. Low-frequency electromagnetic waves also diverge, making them difficult to focus. Large horn antennas and lens antennas have poor focusing effects, resulting in scattered received signals and impacting test accuracy and reliability.

[0009] The application of Luneburg lenses can effectively address these issues. With the development of metamaterials and transformation optics, researchers have begun exploring the use of novel materials and structures to optimize the performance of Luneburg lenses. Lenses designed using metamaterials not only maintain a compact size and lightweight, but also provide high gain across a wide bandwidth, significantly improving the focusing and transmission of electromagnetic waves. They are particularly suitable for low-frequency transmittance testing.

[0010] Currently, the main problems with the transmittance test in the low-frequency band are as follows: The size and weight problems of traditional focusing lens antennas: In the low-frequency band, due to the long wavelength, the focusing lens antenna is large in size and heavy in weight, which makes it difficult to operate and deploy in actual tests, especially when a quick response is required or in an environment with limited space. High manufacturing costs: The manufacture of focusing lens antennas involves complex processes and high-quality materials, resulting in high costs, which limits its widespread use in economically sensitive applications. Testing flexibility and economic issues: Due to the size and weight limitations of the focusing lens antenna, the testing process lacks flexibility, and the high cost also affects the economic feasibility of the test. Testing accuracy and efficiency issues: The existing technology has low testing accuracy and efficiency in the low-frequency band and cannot quickly and accurately evaluate the material's wave transmission performance. Summary of the Invention

[0011] The purpose of the present invention is to overcome the problem of how to effectively test the transmittance of materials in the low frequency band (300MHz to 2GHz), and proposes a method for testing the transmittance of materials at low frequencies based on Luneburg lenses.

[0012] In order to achieve the above object, the present invention adopts the following technical solutions: In a first aspect, the present invention provides a method for testing the wave transmittance of a material at low frequency based on a Luneburg lens, comprising the following steps: A test fixture, a transmitting Luneburg lens antenna, and a receiving Luneburg lens antenna are placed on a test platform. A material sample to be tested is placed on the test fixture, and the material sample to be tested is placed between the transmitting Luneburg lens antenna and the receiving Luneburg lens antenna. A test distance is adjusted so that the center of the material sample to be tested is at a focal position of the transmitting Luneburg lens antenna and the receiving Luneburg lens antenna. The transmitting Luneburg lens antenna and the receiving Luneburg lens antenna are respectively connected to a vector network analyzer. The transmitting Luneburg lens antenna comprises a transmitting antenna and a transmitting-side Luneburg lens provided at a transmitting port of the transmitting antenna; The receiving Luneburg lens antenna comprises a receiving antenna and a receiving-side Luneburg lens provided at a receiving port of the receiving antenna; The electromagnetic wave is emitted by the transmitting antenna, focused by the Luneburg lens on the transmitting side to form a parallel wave, which then penetrates the material sample to be tested, enters the Luneburg lens on the receiving side and is focused and received by the receiving antenna. A vector network analyzer is used to compare the transmission signal parameters of the transmitting antenna with the reception signal parameters of the receiving antenna to evaluate the wave transmittance of the material sample under test at low frequency.

[0013] Furthermore, the transmitting antenna and the receiving antenna are horn antennas.

[0014] Furthermore, the relative dielectric constants of the transmitting-side Luneburg lens and the receiving-side Luneburg lens satisfy the following relationship:

[0015] in, ε r is the relative permittivity, r is the radial distance from the center of the Luneburg lens to any point, R is the radius of the Luneburg lens.

[0016] Furthermore, the refractive indexes of the transmitting-side Luneburg lens and the receiving-side Luneburg lens are expressed as follows:

[0017] in, n is the refractive index, r is the radial distance from the center of the Luneburg lens to any point, R is the radius of the Luneburg lens.

[0018] Furthermore, the diameters of the transmitting-side Luneburg lens and the receiving-side Luneburg lens are determined according to the following relationship:

[0019] in, D is the diameter of the Luneburg lens, d is the distance between the transmitting antenna and the receiving antenna, λ is the test frequency wavelength.

[0020] Furthermore, the transmission parameters of the transmitting antenna and the receiving parameters of the receiving antenna include transmission intensity, transmission phase, reception intensity and reception phase.

[0021] Furthermore, the low frequency is 300 MHz to 2 GHz.

[0022] Furthermore, the vector network analyzer converts the frequency domain signal into a time domain signal, performs time domain gate filtering, and then converts the time domain signal after the time domain gate filtering into a frequency domain signal; The time domain gate filtering is to set a time window in the time domain, retaining only part of the waveform of the target signal and excluding the clutter signal.

[0023] Furthermore, without placing the material sample to be tested, the transmitting Luneburg lens antenna and the receiving Luneburg lens antenna are used to perform tests under air conditions, and the parameter values ​​are recorded as a reference benchmark for the transmittance test of the material sample to be tested.

[0024] In a second aspect, the present invention provides a low-frequency material transmittance testing device based on a Luneburg lens, comprising a transmitting Luneburg lens antenna, a receiving Luneburg lens antenna, a test fixture, and a test platform, wherein the transmitting Luneburg lens antenna, the receiving Luneburg lens antenna, and the test fixture are placed on the test platform; The test fixture is placed between the transmitting Luneburg lens antenna and the receiving Luneburg lens antenna; The material sample to be tested is placed on the test fixture; The transmitting Luneburg lens antenna and the receiving Luneburg lens antenna are respectively connected to a vector network analyzer; The transmitting Luneburg lens antenna comprises a transmitting antenna and a transmitting-side Luneburg lens provided at a transmitting port of the transmitting antenna; The receiving Luneburg lens antenna comprises a receiving antenna and a receiving-side Luneburg lens provided at a receiving port of the receiving antenna; The transmitting port of the transmitting antenna and the receiving port of the receiving antenna are arranged opposite to the center of the material sample to be tested.

[0025] Compared with the prior art, the present invention has the following beneficial technical effects: The present invention proposes a method for testing the transmittance of materials at low frequencies based on Luneburg lenses. By installing two Luneburg lenses at the transmitting and receiving ports of an ordinary low-frequency horn antenna, the focusing characteristics of the Luneburg lenses are utilized to effectively achieve the focusing of electromagnetic waves, thereby obtaining a narrower beam. In this way, the test signal can be more concentratedly focused on the surface of the material to be tested, thereby improving the signal strength and accuracy of the test. Compared with this test method, the focusing lens at low frequencies is expensive, bulky in size and heavy in weight, making low-frequency testing extremely inconvenient. The low-frequency band test solution provided by the present application has higher flexibility, lower manufacturing costs, and stronger environmental adaptability, and can be widely used in various test scenarios.

[0026] Furthermore, the present invention successfully solves the technical bottleneck in low-frequency band testing by installing two Luneburg lenses on the transmitting and receiving ports of an ordinary low-frequency horn antenna and utilizing the wave-focusing ability of the Luneburg lens. The Luneburg lens, through its unique spherically symmetric refractive index distribution structure, accurately focuses the divergent electromagnetic waves emitted by the horn antenna into parallel waves, thereby significantly improving the energy concentration of the electromagnetic waves, allowing low-frequency signals to more effectively pass through the material to be tested in far-field testing. The transmitted signal is refocused by the receiving end, and the system can achieve high-precision measurement of key parameters such as signal strength and phase. In addition, because the Luneburg lens is more compact in structural design, it reduces the space occupied and system weight of traditional large-size antennas, greatly improving the flexibility and economy of low-frequency band transmittance testing.

[0027] Furthermore, compared to traditional high-frequency testing methods, this invention combines a Luneburg lens with a low-frequency horn antenna, effectively addressing the challenges of large low-frequency testing systems, severe signal divergence, and high testing distance requirements. Leveraging the wave-focusing properties of the Luneburg lens, this invention achieves efficient focusing and transmission of signal energy, significantly improving the accuracy, efficiency, and ease of use of low-frequency material transmittance testing. This provides an innovative, economical, and efficient solution for testing low-frequency wave-transmitting materials.

[0028] Furthermore, the present invention overcomes the challenges of high manufacturing costs, fixed size limitations, high equipment investment, complex operation and maintenance, and stability and convenience issues in low-frequency testing. By providing an innovative testing solution, the present invention enhances testing flexibility, accuracy, and applicability while reducing costs and operational complexity, ensuring effective testing of samples of various sizes, particularly at low frequencies, to meet the needs of diverse technical research. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The drawings described herein are for illustrative purposes only and are not intended to limit the scope of the present invention in any way. In addition, the shapes and proportional dimensions of the components in the drawings are only schematic and are used to help understand the present invention, and are not intended to specifically limit the shapes and proportional dimensions of the components of the present invention. In the drawings: Figure 1 This is a structural diagram of a low-frequency material transmittance testing device based on a Luneburg lens.

[0030] Figure 2 This is the schematic diagram of the Luneburg lens emission principle.

[0031] Figure 3 This is the receiving principle diagram of the Luneburg lens.

[0032] Figure 4 is the spectrum of the unfiltered signal.

[0033] Figure 5 is the signal spectrum after time domain gated filtering.

[0034] Among them, 1 is the transmitting antenna, 2 is the transmitting side Luneburg lens, 3 is the material sample to be tested, 4 is the receiving side Luneburg lens, and 5 is the receiving antenna. DETAILED DESCRIPTION

[0035] In order to enable those skilled in the art to better understand the solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts should fall within the scope of protection of the present invention.

[0036] It should be noted that when an element is referred to as being "disposed on" another element, it may be directly on the other element or there may be an element centered thereon. When an element is referred to as being "connected to" another element, it may be directly connected to the other element or there may be an element centered thereon. The terms "vertical," "horizontal," "left," "right," and similar expressions used herein are for illustrative purposes only and do not represent the only embodiments.

[0037] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one skilled in the art to which this invention pertains. The terms used in this specification of the present invention are for the purpose of describing specific embodiments only and are not intended to limit the present invention. The term "and / or" as used herein includes any and all combinations of one or more of the associated listed items.

[0038] It should be noted that the terms "first", "second", etc. in the description and claims of the present invention and the above-mentioned drawings are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that the numbers used in this way can be interchanged where appropriate, so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions. For example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0039] Example 1 A method for testing the wave transmittance of a material at low frequency based on a Luneburg lens comprises the following steps: A test fixture, a transmitting Luneburg lens antenna, and a receiving Luneburg lens antenna are placed on a test platform. A material sample 3 to be tested is placed on the test fixture. The material sample 3 to be tested is placed between the transmitting Luneburg lens antenna and the receiving Luneburg lens antenna. The test distance is adjusted so that the center of the material sample 3 to be tested is at the focal position of the transmitting Luneburg lens antenna and the receiving Luneburg lens antenna. The transmitting Luneburg lens antenna and the receiving Luneburg lens antenna are respectively connected to a vector network analyzer. The transmitting Luneburg lens antenna comprises a transmitting antenna 1 and a transmitting-side Luneburg lens 2 provided at a transmitting port of the transmitting antenna 1; The receiving Luneburg lens antenna includes a receiving antenna 5 and a receiving-side Luneburg lens 4 provided at a receiving port of the receiving antenna 5; The electromagnetic wave is emitted by the transmitting antenna 1, focused by the transmitting side Luneburg lens 2 to form a parallel wave, penetrates the material sample 3 to be tested, enters the receiving side Luneburg lens 4 to be focused, and is received by the receiving antenna 5; The vector network analyzer is used to compare the transmission signal parameters of the transmitting antenna 1 with the reception signal parameters of the receiving antenna 5 to evaluate the wave transmittance of the material sample 3 under low frequency.

[0040] This embodiment utilizes the wave focusing ability of the Luneburg lens to test the material transmittance, solving the problem of low-frequency testing. By adjusting the position of the mounting platform, precise focusing of the Luneburg lens antenna and the test piece is achieved. Combining simulation and experimental data, the test method is optimized to improve the accuracy and reliability of the test. Compared with traditional methods, it is easy to operate. By installing the Luneburg lens on the ordinary antenna port, the test process is simplified and the test cost is reduced. The high manufacturing cost of large focusing lenses at low frequencies is avoided, making the test more economical and efficient. It is suitable for transmittance testing from low to high frequency bands, which expands the application range of the test. The method of the present invention can be quickly deployed and tested, and is suitable for testing needs in various environments and conditions.

[0041] The transmitting antenna 1 and the receiving antenna 5 are horn antennas.

[0042] The relative dielectric constants of the transmitting-side Luneburg lens 2 and the receiving-side Luneburg lens 4 satisfy the following relationship:

[0043] in, ε r is the relative permittivity, r is the radial distance from the center of the Luneburg lens to any point, R is the radius of the Luneburg lens.

[0044] The refractive indexes of the transmitting-side Luneburg lens 2 and the receiving-side Luneburg lens 4 are expressed as follows:

[0045] in, n is the refractive index, r is the radial distance from the center of the Luneburg lens to any point, R is the radius of the Luneburg lens.

[0046] The diameters of the transmitting-side Luneburg lens 2 and the receiving-side Luneburg lens 4 are determined according to the following relationship:

[0047] in, D is the diameter of the Luneburg lens, d is the distance between the transmitting antenna and the receiving antenna, λ is the test frequency wavelength.

[0048] The transmission parameters of the transmitting antenna 1 and the receiving parameters of the receiving antenna 5 include transmission intensity, transmission phase, reception intensity and reception phase.

[0049] The low frequency is 300MHz to 2GHz.

[0050] The vector network analyzer converts the frequency domain signal into a time domain signal, performs time domain gate filtering, and then converts the time domain signal after time domain gate filtering into a frequency domain signal; Time domain gate filtering sets a time window in the time domain to retain only part of the waveform of the target signal and exclude the clutter signal.

[0051] Without placing the material sample 3 to be tested, the transmitting Luneburg lens antenna and the receiving Luneburg lens antenna are used to perform the test under air conditions, and the parameter values ​​are recorded as a reference benchmark for the transmittance test of the material sample 3 to be tested.

[0052] Example 2 See also Figure 1 A Luneburg lens-based low-frequency material transmittance test device includes a transmitting Luneburg lens antenna, a receiving Luneburg lens antenna, a test fixture, and a test platform. The transmitting Luneburg lens antenna, the receiving Luneburg lens antenna, and the test fixture are placed on the test platform. The test fixture is placed between the transmitting Luneburg lens antenna and the receiving Luneburg lens antenna; A sample of the material to be tested 3 is placed on the test fixture; The transmitting Luneburg lens antenna and the receiving Luneburg lens antenna are respectively connected to a vector network analyzer; The transmitting Luneburg lens antenna comprises a transmitting antenna 1 and a transmitting-side Luneburg lens 2 provided at a transmitting port of the transmitting antenna 1; The receiving Luneburg lens antenna includes a receiving antenna 5 and a receiving-side Luneburg lens 4 provided at a receiving port of the receiving antenna 5; The transmitting port of the transmitting antenna 1 and the receiving port of the receiving antenna 5 are arranged opposite to the center of the material sample 3 to be tested.

[0053] Example 3 This embodiment provides a method for testing the wave transmittance of materials at low frequencies based on Luneburg lenses. The structural diagram of this testing method is shown in FIG. Figure 1As shown, a transmitting antenna 1 and a receiving antenna 5 are mounted on the left and right sides, respectively, with a material sample 3 under test placed in the center. Luneburg lenses are installed at the transmitting and receiving ports of the two horn antennas to focus the transmitted and received electromagnetic waves. This innovative design significantly improves the interaction efficiency between the electromagnetic waves and the material under test by precisely controlling the propagation direction of the electromagnetic waves, thereby enhancing the accuracy of transmittance testing. This is particularly suitable for addressing critical issues in low-frequency testing between 300 MHz and 2 GHz. After determining the appropriate testing distance, two Luneburg lenses are mounted at the ports of the transmitting antenna 1 and the receiving antenna 5, respectively. The Luneburg lenses' wave-focusing capabilities focus the electromagnetic wave energy onto the material sample 3 under test, enabling more accurate measurement of transmittance performance. The material sample 3 under test is precisely positioned at the focal point of the Luneburg lenses. When the electromagnetic wave penetrates the material sample 3, the receiving antenna 5 captures the transmitted signal. By comparing the intensity and phase of the transmitted and received signals, the sample's transmittance, or its ability to transmit electromagnetic waves, can be accurately assessed.

[0054] Traditional focusing lenses typically concentrate electromagnetic waves into a small area. This can result in uneven illumination across large test samples, leading to uneven test results and errors. Luneburg lenses, by contrast, utilize a unique design to focus electromagnetic waves over a wider area, ensuring uniform distribution of the test signal across the sample surface and improving test accuracy and consistency. Furthermore, Luneburg lenses effectively convert electromagnetic waves into parallel waves, reducing signal scattering and attenuation during propagation, further enhancing test signal stability and reliability. Compared to traditional lenses, which require larger equipment due to their longer wavelengths in low-frequency bands, Luneburg lenses not only achieve a wider focusing range in a smaller footprint, but also offer a more lightweight and convenient design, making them particularly suitable for low-frequency transmittance testing, ensuring accuracy while reducing equipment size and weight. Therefore, compared to traditional lenses, Luneburg lenses offer higher test accuracy, a wider focusing range, and greater flexibility and ease of operation and deployment in low-frequency transmittance testing.

[0055] 1. Luneburg lens antenna The design of the Luneburg lens antenna is inspired by the concept of "lens" in optics. Its basic principle is to focus electromagnetic waves through a spherical structure with a continuously changing refractive index. This enables the Luneburg lens to play an important role in many fields such as radar, communications, and radio astronomy. The schematic diagram of the Luneburg sphere lens is as follows: Figure 2 and Figure 3As shown in Figure 2, its operating principle is based on the gradient variation of the refractive index within the lens along the radius of a sphere. This gradual change in refractive index from the outside to the inside of the lens creates a spherically symmetrical structure, effectively focusing incident electromagnetic waves to a single point on the sphere's surface or converting spherical waves from a point source into parallel waves. Specifically, the relative dielectric constant of a Luneburg lens is highest at the lens center, while at the lens surface, it approaches the refractive index of air, approximately 1. This unique structure allows Luneburg lenses to focus incident plane waves to a focal point on the other side of the lens or to converge spherical waves from a point source into parallel waves, thereby achieving precise focusing and propagation of electromagnetic waves. This unique design gives Luneburg lenses significant advantages in electromagnetic wave focusing and beam conversion, making them widely used in applications requiring high directivity and focusing accuracy. A Luneburg lens is a spherical lens composed of dielectric materials with varying dielectric constants, primarily used for focusing and directing electromagnetic waves. Its design principle is based on the variation in refractive index. By precisely controlling the dielectric constant and thickness of each dielectric layer, electromagnetic waves propagating at different angles are focused to a single point on the lens surface. Unlike traditional focusing lenses, the design of the Luneburg lens precisely controls the focusing characteristics of electromagnetic waves through radially varying dielectric constants, thereby achieving precise irradiation of specific areas.

[0056] A key design element of a Luneburg lens is its dielectric constant distribution. The dielectric constant decreases from the outer surface to the inner layer of the lens. At the surface, the dielectric constant is close to 1, while in the center of the lens, the dielectric constant reaches a higher value, such as 2. This distribution allows the Luneburg lens to focus electromagnetic waves from different directions onto a single point on the lens surface, forming a focal point. The specific distribution is given by Formula 1: 1 in, ε r is the relative permittivity, r is the radial distance from the center of the lens to any point, R is the radius of the lens. This formula describes how the dielectric constant varies with the radial direction, i.e. the dielectric constant is smaller near the lens surface and larger in the center of the lens.

[0057] In a Luneburg lens, the change in refractive index is the core mechanism for focusing electromagnetic waves. According to the law of refraction, when an electromagnetic wave passes through a medium with a different refractive index, its propagation direction changes. The Luneburg lens achieves precise control of electromagnetic waves by adjusting the dielectric constant, that is, the radial variation of the refractive index. The refractive index of the Luneburg sphere lens varies according to Formula 2: 2 When a plane electromagnetic wave strikes the "lens," it is focused by the lens to the other end of the diameter perpendicular to the plane electromagnetic wave. Placing a feed at this point, typically the focal point, converts it into a plane electromagnetic wave at the spherical antenna's aperture. Simply moving the feed across the spherical surface allows for 360° beam scanning.

[0058] The Luneburg lens is designed to focus the electromagnetic waves emitted by the horn antenna at the emission port. The electromagnetic waves emitted by the horn antenna itself usually radiate in all directions and have a certain degree of divergence. The Luneburg lens focuses these originally divergent electromagnetic waves into parallel waves through the design of its internal dielectric layer, thereby enhancing the propagation directionality of the electromagnetic waves and ensuring that they can be emitted more concentratedly to the material to be tested. This focused electromagnetic wave has a very narrow beam width, which can interact with the material more accurately and provide more accurate transmittance test results. The beam width after focusing, usually called the beam half-power point width or main lobe width, is a key parameter that describes the spatial distribution of the output signal of a focused antenna such as a Luneburg lens antenna. It represents the width of the beam, that is, the angular width when the signal intensity drops to 50% of the maximum value. The width of the focused beam is affected by many factors, including the frequency of the source signal, the size and shape of the lens, and the optical design parameters. In an ideal case, the width of the focused beam can be estimated by the diffraction limit. For an ideal lens antenna such as a Luneburg lens, the beam width 𝜃 is proportional to the effective aperture D of the lens and the wavelength of the signal. λ The relationship between is usually given by Formula 3: 3 in: 𝜃 is the half-power point width of the beam, usually expressed in radians, which is the angular width of the beam.

[0059] D is the effective aperture diameter of the lens.

[0060] For Luneburg lenses, the width of the focused beam can be understood by the relationship between the refractive index distribution and the lens aperture. Ideally, Luneburg lenses, through their unique refractive index distribution—the radial variation of the dielectric constant—can more precisely focus incident electromagnetic waves into a small area, resulting in a smaller focused beam than conventional lenses. Therefore, Luneburg lenses generally offer superior focusing performance compared to traditional simple lenses, enabling narrower beams.

[0061] In order to ensure that the electromagnetic waves are focused and form parallel waves, the size 𝐷 of the Luneburg lens needs to meet the far-field condition. According to the far-field condition formula, the minimum size of the Luneburg lens is D It can be estimated by formula 4: 4 Therefore, in low-frequency testing, the dimensions of the Luneburg lens should meet the following ranges within the 300 MHz to 2 GHz frequency band: The minimum dimension is D = 0.547 m for 2 GHz.

[0062] Maximum dimension is D=1m for 300 MHz Therefore, the size of the Luneburg lens needs to be reasonably selected according to the specific test frequency, space limitations and test requirements to ensure that efficient and accurate transmittance test results can be obtained.

[0063] 2. Tested samples The size of the sample to be tested should be large enough to cover the wavelength requirements within the test frequency range to ensure the effectiveness of the test. For tests in low frequency bands such as 300 MHz to 2 GHz, the size of the sample needs to be larger than or close to the wavelength by 100 cm to 15 cm. The size of the sample to be tested must be at least 2.5 times the wavelength of the test frequency band to avoid unrepresentative or inaccurate test results due to too small a size. At the same time, the sample should have a regular shape to facilitate installation and precise positioning, ensuring that the propagation of electromagnetic waves on the sample is not subject to additional interference caused by irregular shapes. Edge effects are a common problem that affects transmittance test results, so samples should try to avoid irregular edges to reduce the impact of this effect on the test results.

[0064] The sample surface should be flat, undamaged, and free of contamination to minimize interference with the test signal. For samples that need to be rotated, the projected area of ​​the sample should be within the required range during rotation to minimize the impact of edge diffraction.

[0065] The sample should have uniform material properties, whether it is wave transmission, wave absorption or reflection, to ensure the accuracy of the test results.

[0066] During testing, samples should maintain good physical and chemical stability to prevent volatilization, decomposition, or oxidation from affecting test results. High-frequency electromagnetic radiation can also affect samples due to environmental factors such as temperature fluctuations and humidity, leading to fluctuations in material properties. Therefore, during testing, samples should be measured under stable environmental conditions, and appropriate measures, such as temperature and humidity control, should be implemented to prevent environmental fluctuations from interfering with test results.

[0067] 3. Test distance problem When using Luneburg lens antennas to test material transmittance at low frequencies, determining the correct test distance is crucial to the accuracy of the test results. Unlike traditional far-field testing methods, Luneburg lens antennas are typically tested using near-field testing. In this method, the test distance selection prioritizes the lens's focusing effect and the accuracy of signal transmission.

[0068] Due to the focusing effect of the Luneburg lens, it can concentrate electromagnetic waves into a small area, making the test distance relatively short. However, if the test distance is too long, the electromagnetic waves will experience significant losses during propagation, especially in the low-frequency range of 300 MHz to 2 GHz, where the wavelength is longer and the signal attenuation and diffusion are more significant. This excessive propagation distance will result in a sparser energy distribution of the beam, significantly reducing the signal strength received by the receiver, which in turn affects the accuracy of the transmittance test and the reliability of the results.

[0069] In actual testing, it's usually necessary to maintain a moderate distance between the antennas and the sample. This is necessary to avoid both the near-field effect that can affect the results and excessive signal attenuation caused by a long distance. A reasonable test distance ensures that the Luneburg lens's focusing effect is fully utilized while minimizing electromagnetic wave losses during propagation, thereby ensuring accurate and repeatable test results.

[0070] 4. Time Domain Gate Filtering When conducting wave transmittance tests, the test environment often presents a significant amount of clutter and interference signals. In frequency domain analysis, this clutter can overlap with the target signal's spectrum, making it difficult to separate and identify the signals. This clutter can originate from the device's own noise, environmental noise, or other RF sources. In traditional frequency domain measurements, the intertwining of clutter and the test signal's spectrum makes it difficult to filter out the clutter directly in the frequency domain. Therefore, to effectively improve test accuracy, time domain analysis methods are required to address this issue.

[0071] To effectively remove clutter, the time domain conversion function of a vector network analyzer can be utilized. Specifically, the vector network analyzer first converts the frequency domain signal into the time domain. In the time domain, the waveform of the target signal often exhibits specific characteristics, such as distinct peaks and signal duration characteristics. However, clutter signals, which typically exist as noise, lack these distinct time domain characteristics. Therefore, time domain analysis makes it easier to identify valid signals and distinguish them from clutter.

[0072] In time-domain signals, time-domain gating can be applied to accurately extract the target signal and filter out clutter. By setting a "gated" region in the time domain—defining a time window—only a portion of the target signal waveform is retained, while clutter signals are excluded. Specifically, the time-domain gate is typically selectively opened during the valid portion of the signal, allowing only the target signal to pass through. Clutter signals, which fall outside this time window, are automatically suppressed. After completing time-domain gating, the filtered time-domain signal can be converted back to the frequency domain, yielding more accurate frequency-domain test data. This method effectively reduces the impact of clutter on frequency-domain analysis, improving the accuracy and reliability of transmittance testing.

[0073] The comparison of test results before and after filtering is as follows: Figure 4 and Figure 5 shown. Figure 4 Before filtering, Figure 5 After filtering. The unfiltered signal spectrum contains significant noise and clutter components, which cause irregular fluctuations in the spectrum and affect the accuracy and identifiability of the target signal. After time-domain gated filtering, the clutter components in the spectrum are effectively suppressed, the signal's frequency response becomes smoother, and the frequency distribution of the clutter is removed, making the target signal's main frequency component more prominent and the overall shape of the spectrum more standardized.

[0074] 5. Testing Process Preheat the vector network analyzer: Preheat the vector network analyzer for 15 to 30 minutes to ensure stable operation of the instrument.

[0075] Connect the test equipment: Figure 1 Connect the test equipment and ensure that the connections between the test equipment are correct.

[0076] Set instrument parameters: Set the frequency range, number of measurement frequency points, intermediate frequency bandwidth, and output power of the vector network analyzer. If necessary, set the time domain window range to improve test accuracy.

[0077] Align the transceiver antenna: Use a laser to align the axis of the transceiver antenna to ensure the correct direction of the antenna.

[0078] Place the test fixture: Place the test fixture at the focal point of the two Luneburg lens antennas, ensuring that the distance between the test fixture and the transmit and receive antenna apertures is consistent. At this point, the Luneburg lens size should be selected based on the test frequency range, ensuring that the focal point is at the appropriate test distance. Install the Luneburg lens in front of the horn antenna.

[0079] Check the test piece: Check whether the surface condition and flatness of the material meet the requirements, and calculate whether the size of the test piece meets the standards.

[0080] Air Calibration: Without placing the test material, use two Luneburg lens antennas to perform the test in air conditions. Record the S21 parameter value at this time and use it as a reference for subsequent measurements and further calibration corrections.

[0081] Adjust the antenna and DUT positions: Adjust the aperture of the transceiver antenna to ensure it is in the primary polarization state. Place the DUT on the test fixture, ensuring that the DUT is perpendicular to the antenna axis and that the Luneburg lens focus is at the center of the DUT.

[0082] Record test data: Perform tests as required and record the test data of the vector network analyzer.

[0083] Complete the test and reset: After the test is completed, reset the turntable, turn off the vector network analyzer, remove the device under test, and restore the items in the test system.

[0084] Many embodiments and applications beyond the examples provided will be apparent to those skilled in the art upon reading the foregoing description. Therefore, the scope of the present teachings should be determined not with reference to the foregoing description, but rather with reference to the preceding claims, along with the full scope of equivalents to which such claims are entitled. For the purpose of completeness, all articles and references, including the disclosures of patent applications and publications, are incorporated herein by reference. The omission of any aspect of the subject matter disclosed herein from the preceding claims is not a disclaimer of such subject matter, nor should it be interpreted that the applicants did not consider such subject matter to be part of the disclosed inventive subject matter.

[0085] The above content is a further detailed description of the present invention, and it cannot be considered that the specific implementation methods of the present invention are limited to these. For ordinary technicians in the technical field to which the present invention belongs, they can make several simple deductions or substitutions without departing from the concept of the present invention, which should be regarded as belonging to the determined protection scope of the present invention submitted.

Claims

1. A method for testing the wave transmittance of materials at low frequencies based on Luneburg lenses, characterized in that: The following steps are involved: A test fixture, a transmitting Luneburg lens antenna, and a receiving Luneburg lens antenna are placed on a test platform, a material sample (3) to be tested is placed on the test fixture, the material sample (3) to be tested is placed between the transmitting Luneburg lens antenna and the receiving Luneburg lens antenna, and a test distance is adjusted so that the center of the material sample (3) to be tested is at a focal position of the transmitting Luneburg lens antenna and the receiving Luneburg lens antenna; The transmitting Luneburg lens antenna and the receiving Luneburg lens antenna are respectively connected to a vector network analyzer; The transmitting Luneburg lens antenna comprises a transmitting antenna (1) and a transmitting-side Luneburg lens (2) arranged at a transmitting port of the transmitting antenna (1); The receiving Luneburg lens antenna comprises a receiving antenna (5) and a receiving-side Luneburg lens (4) arranged at a receiving port of the receiving antenna (5); Electromagnetic waves are emitted through a transmitting antenna (1), focused by a transmitting side Luneburg lens (2) to form parallel waves, penetrate a sample of a material to be tested (3), enter a receiving side Luneburg lens (4), are focused, and are received by a receiving antenna (5); A vector network analyzer is used to compare the transmission signal parameters of the transmitting antenna (1) and the reception signal parameters of the receiving antenna (5) to evaluate the wave transmittance of the material sample (3) under low frequency.

2. The method for testing the wave transmittance of a material at low frequency based on a Luneburg lens according to claim 1, characterized in that: The transmitting antenna (1) and the receiving antenna (5) are horn antennas.

3. The method for testing the wave transmittance of a material at low frequency based on a Luneburg lens according to claim 1, characterized in that: The relative dielectric constants of the transmitting-side Luneburg lens (2) and the receiving-side Luneburg lens (4) satisfy the following relationship: in, ε r is the relative permittivity, r is the radial distance from the center of the Luneburg lens to any point, R is the radius of the Luneburg lens.

4. The method for testing the wave transmittance of a material at low frequency based on a Luneburg lens according to claim 1, characterized in that: The refractive indexes of the transmitting-side Luneburg lens (2) and the receiving-side Luneburg lens (4) are shown in the following formula: in, n is the refractive index, r is the radial distance from the center of the Luneburg lens to any point, R is the radius of the Luneburg lens.

5. The method for testing the wave transmittance of a material at low frequency based on a Luneburg lens according to claim 1, wherein: The diameters of the transmitting-side Luneburg lens (2) and the receiving-side Luneburg lens (4) are determined according to the following relationship: in, D is the diameter of the Luneburg lens, d is the distance between the transmitting antenna and the receiving antenna, λ is the test frequency wavelength.

6. The method for testing the wave transmittance of a material at low frequency based on a Luneburg lens according to claim 1, characterized in that: The transmission parameters of the transmitting antenna (1) and the receiving parameters of the receiving antenna (5) include transmission intensity, transmission phase, receiving intensity and receiving phase.

7. The method for testing the wave transmittance of a material at low frequency based on a Luneburg lens according to claim 1, characterized in that: The low frequency is 300 MHz to 2 GHz.

8. The method for testing the wave transmittance of a material at low frequency based on a Luneburg lens according to claim 1, characterized in that: The vector network analyzer converts the frequency domain signal into a time domain signal, performs time domain gate filtering, and then converts the time domain signal after the time domain gate filtering into a frequency domain signal; The time domain gate filtering is to set a time window in the time domain, retaining only part of the waveform of the target signal and excluding the clutter signal.

9. The method for testing the wave transmittance of a material at low frequency based on a Luneburg lens according to claim 1, wherein: Without placing the material sample (3) to be tested, the transmitting Luneburg lens antenna and the receiving Luneburg lens antenna are used to conduct the test under air conditions, and the parameter values ​​are recorded as a reference benchmark for the transmittance test of the material sample (3) to be tested.

10. A low-frequency material transmittance test device based on Luneburg lens, characterized in that: It includes a transmitting Luneburg lens antenna, a receiving Luneburg lens antenna, a test fixture and a test platform, wherein the transmitting Luneburg lens antenna, the receiving Luneburg lens antenna and the test fixture are placed on the test platform; The test fixture is placed between the transmitting Luneburg lens antenna and the receiving Luneburg lens antenna; The material sample to be tested (3) is placed on the test fixture; The transmitting Luneburg lens antenna and the receiving Luneburg lens antenna are respectively connected to a vector network analyzer; The transmitting Luneburg lens antenna comprises a transmitting antenna (1) and a transmitting-side Luneburg lens (2) arranged at a transmitting port of the transmitting antenna (1); The receiving Luneburg lens antenna comprises a receiving antenna (5) and a receiving-side Luneburg lens (4) arranged at a receiving port of the receiving antenna (5); The transmitting port of the transmitting antenna (1) and the receiving port of the receiving antenna (5) are arranged relative to the center of the material sample (3) to be tested.

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